The aerobic threshold is a term usually used to refer to a level of exercise below the anaerobic threshold (AeT). It is considered to be the point at which the supply of oxygen is sufficient fuel for the muscles, and the anaerobic energy pathways have just begun to kick in. This occurs at about 65% of the maximum heart rate. The anaerobic threshold is 40 beats faster. During aerobic exercise, the body takes in oxygen, which it uses to fuel the muscles. The anaerobic threshold is reached when there is no longer enough oxygen and the body starts to produce more lactic acid.

Aerobic means "with air," while anaerobic means "without air." Some forms of exercise like walking and dancing are aerobic because they cause the heart to beat faster and breathing to increase, to help deliver oxygen to the muscles. There is no such thing as aerobic threshold because there is no physiological change occurring, just the aerobic system working as it should.

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When an athlete starts to feel out of breath or winded, this is when the body can no longer supply enough oxygen and the muscles begin to work in an oxygen-deprived state. Lactic acid starts to build up in the muscles. It was once believed that the buildup of lactic acid was a metabolic waste product that was detrimental to the athlete and led to muscle fatigue and soreness. For this reason, athletes were trained at just below their anaerobic threshold. More recent studies conclude that the lactic acid is there to be used as a fuel, much like carbohydrates in muscle tissue and sugar in the blood stream.

An athlete who has worked out using endurance training will have taught his body to use the lactic acid efficiently as a fuel and get rid of it before it can reach the point where it causes muscle fatigue. Endurance training comprises an assortment of stop-start exercises, which demand sudden bursts of energy. This does not allow enough time for oxygen to reach the muscle, so the muscle must find another source of fuel, which is lactic acid.

Threshold training is what athletes who perform in endurance sports such as cycling and long distance running often undergo. While they train, they check their heart-rate monitors in order to ensure their pulse rate stays within a certain range or they are measuring their aerobic threshold. This is defined as the optimum level at which the athlete is still working within his aerobic zone. Targeting an aerobic threshold means that the athlete's lungs, heart, and other key systems are able to perform efficiently for long periods of time. Another definition of aerobic threshold is the intensity at which the athlete can perform for hours on end.

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